1. Technical Field
The present invention relates to a wavelength-variable interference filter that selectively outputs light of a desired wavelength from incident light, an optical module that has the wavelength-variable interference filter, and an optical analysis device that has the optical module.
2. Related Art
A wavelength-variable interference filter has been known in which reflective films are disposed on opposite surfaces of a pair of substrates with a gap of a predetermined size interposed therebetween (for example, see JP-A-11-142752).
In the wavelength-variable interference filter described in JP-A-11-142752, driving electrodes are disposed on opposite surfaces of a pair of reflective films so as to adjust the gap, and the gap can be adjusted by the use of an electrostatic attractive force by applying a drive voltage to the driving electrodes. Accordingly, the wavelength-variable interference filter can transmit only light of a specific wavelength depending on the gap. That is, the wavelength-variable interference filter causes incident light to multiply interfere between a pair of reflective films and transmits only light of a specific wavelength which is strengthened due to the multiple interference.
Since the wavelength-variable interference filter transmits only light of a desired wavelength by adjusting the gap size, it requires high gap precision.
However, when the reflective film or the driving electrode is formed on one substrate, an internal stress acts in the in-plane direction (direction parallel to the surface of the substrate) of the reflective film and the driving electrode. The magnitude of the internal stress or the direction in which the internal stress acts is determined depending on film formation methods or film materials. The internal stress serves as a compressive stress when it acts in the direction towards each other, and the internal stress serves as a tensile stress when it acts in the direction awary from each other. Here, when the compressive stress acts on the driving electrode or the reflective film formed on one substrate, the substrate warps convexly toward the other substrate. On the other hand, when the tensile stress acts on the driving electrode or the reflective film formed on one substrate, the substrate warps convexly in the direction in which it is separated from the other substrate.
In this way, when the substrate warps due to the internal stress of the driving electrode or the reflective film, the reflective film also warps in an initial state where no driving voltage is applied to the driving electrode, thereby causing deterioration in resolution of the wavelength-variable interference filter.